1. Field of the Invention
The present invention relates to a metallic carrier base material for maintaining the catalyst for exhaust gas purification to be used generally by mounting in the middle of an exhaust gas pipe as a means to purify exhaust gas of automobiles. Here, the "carrier base material" differs from the "carrier" as used in the field of chemistry; it means an implement used to maintain the catalyst supported by the carrier.
2. Description of Prior Art
In order to enlarge the carrier area per unit volume, i.e. to enlarge the effective contact area between the exhaust gas and the exhaust gas purification catalyst per unit volume, and to ensure the lightweight design for the metallic carrier base Material itself, the metallic carrier base material has conventionally consisted of a multi-layer body of spiral or hierarchical form with a number of screenlike vent holes which pass exhaust gas in a certain direction. Planar band materials and corrugated band materials, made of very thin heatresistant steel strips of less than 0.1 mm in thickness, are piled up alternatively to furnish the contact portion. The metallic carrier base material, composed of such multi-layer body material, is accommodated in a metal case with open ends when necessary. The bonding between the band materials themselves in the contact portion of the multi-layer body, or between the band material and the metal case, is performed by brazing, after the formation of the multi-layer body by electron beam welding or laser welding. This is described in the Japanese Provisional Patent Publication Nos. 54-13462, 57-1585, 56-4373, etc. Alternatively, the bonding is preformed by treating the band materials in advance with the brazing material of powdery or boil types.
As with the above-mentioned heat-resistant steel strip, for example, the heat-resistant steel of chrome-aluminum-ferrite type, containing chrome of 15-25 weight % and aluminum of 2-5 weight % is used, whereas the steel strip of this type is not very easily rolled, and the material cost is expensive, because it is necessary to pass it repeatedly through the process of rolling and annealing, until the thin material of the desired thickness, suitable for carrier base material, is obtained.
Also, as shown in the Japanese Provisional Patent Publication No. 62-11547, it is proposed to provide the layer coated with the substantially pure aluminum on a base metal of special composition, but such base metal is not easily available.
On the other hand, the conventional electron beam welding, or laser welding, requires special and expensive devices in the assembling and bonding to the multi-layer body. It is necessary to perform the troublesome and complicated operation such as spot welding at a narrow contact portion or an internal contact portion, and this results in lower productivity.
In the bonding method by brazing, special care has to be taken on the type and arrangement of brazing material or on the heating condition.